you need to setup your /etc/network/interfaces file.
you will probably want something like this for dhcp:
auto eth0
iface eth0 inet dhcp
or this for static ip:
auto eth0
iface eth0 inet static
address 123.123.123.123
netmask 123.123.123.123
gateway 123.123.123.123
if you are using dhcp, then you also need to edit your /etc/passwd file.
find the line that has
dhcp:x:10n:10n::something and change the 10n (where n is a number) to 0.
mine looks like this:
dhcp:x:0:0::/nonexistent:/bin/false
if you are using static ips, then you will need to edit /etc/resolv.conf
and add your dns server addresses in this format:
nameserver 111.111.111.111
nameserver 222.222.222.222